Ecological Studies in South Benfleet Creek with Special Reference to the Amphipod Genus Corophium B.A.*

Reprinted from, THE ESSEX NATURALIST for 1961 Ecological Studies in South Benfleet Creek with Special Reference to the Amphipod Genus Corophium B.A.* This paper is an account of the work done on some ecological aspects of South Benfleet Creek. A description of the Creek is given in the Introduction and a brief account of the methods used in the second section. The ecology of the most important species found in the Creek is described. The Creek is most interesting with regard to the distribution of the two species of Corophium found there, one of which, Corophium arenarium, is a relatively rare species. It has been found that the composition of the substratum, and associated factors such as water content, is the dominant factor in determining the distribution of the two species of Corophium and of the other two most important species, Nereis diversicolor and Scrobicularia plana. The important distinctions between the habitat of Corophium volutator and Corophium arenarium have also been worked out. INTRODUCTION Benfleet Creek South Benfleet Creek (G.R. TQ 79 85) is a small tidal inlet on the north bank of the River Thames about 7 miles from its mouth at Shoeburyness. The Creek is just over 3 miles long, from Canvey Point to South Benfleet Bridge, nearly half a mile wide at its mouth and about 80 yards wide near the bridge. It runs in a S.S.E.-N.N.W. direction between the low-lying land of Canvey Island and Hadleigh Marsh. Because the surrounding land is all below H.W.M.M.T.* level, strong concrete embank- ments have been built along the whole length of the creek, on both banks. It is therefore closely confined to its channel, with no small tributaries of fresh water entering it. There is a flow of salt water in the main channel, even at low water. As can be seen from figure 2, the creek is bordered on either side by extensive saltings. These are finely dissected by narrow, deep, muddy channels, filled at high water. The main channel is deeply incised at low water in its lower reaches where it is too deep and wide to ford. Just above the island, however, the *The material for this paper was prepared while the author was a student at the University College of North Staffordshire. He is now a research student in Marine Zoology at the University of Wales, University College, Swansea. *High Water Mark of Medium Tides. 292 THE ESSEX NATURALIST stream bed opens out and the stream is fordable. The main stream flows round the south side of this island but a small tributary stream is present on the north side, with its source at the western end of the island. HEIGHT IN FEET 26 10 25 DISTANCE IN YARDS Figure la. The profile of the transect across area B. The stations marked on the profile are those given in Tables II and III. PIE1G447 mreeT • 6 4 . COROPHIum 01.V , 6•0111 20161- - 4 2•- as r•• 103 04 68 42 DISTANCE 114 VAMPS Figure lb. The profile of the transect across area G. The form of the stream as described above, is, in part, re- sponsible for the nature of the substratum of the creek at low tide. Although it may be described generally as estuarine mud, the substratum varies greatly in detail, as will be shown later. Away from the main channel, towards High Water Mark, the muds are very soft and sticky. Nearer the main channel, and at intervals along its whole length, one finds well-defined areas of relatively clean sand which are quite firm to walk on. These 'sand banks' occur usually where there is a small meander in the stream. Just after the turn of the tide the incoming currents are swift (3 ft./ sec.) in the incised channel, too swift to allow for the deposition of the silts but sufficiently slow to allow for the deposition of the sand. As the tide rises the main stream overflows its incised channel, the current is checked and slackens, thus allowing for the deposition of the finer material. This is well SOUTH BENFLEET ...... 4.1•(' '''''' • i''''''''' a 0 Os •Ni• ''''''' •••••••••••••••••• ''''''' G III II •M•lIl 0 0o • ''''''''''' 0 0 - 0 0 a • • LEIGH ISLAND ,•■••• ammi CHANNEL AT L.W.M.T. =1 STONE EMBANKMENT • jttill.fe 1.6.811 EARTH EMBANKMENT 0 bot, • 0 o• COROPHIUM VOLUTATOR cf• 0 0 COROPHIUM ARENARIUM CANVEY ISLAND SHELL DEPOSITS 220 440 880 YARDS. SALTING Figure 2. A map of South Benfleet Creek showing the distribution of C. voNtator and 0. arenarium and the positions of the sampling stations. Enlarged and modified from the Ordnance Survey two and half inch sheet. ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 293 illustrated in Area G (see fig. lb.) where the steeper slope up from the stream appears to be of a sandy nature and much firmer as compared with the flat top of the region, which is of a decidedly muddier consistency, Moore and Spooner (1940), Percival (1929), Rees (1940). The preliminary investigations of the lower reaches of the creek in June revealed the presence of a large population of the amphipod crustacean Corophium. An investigation of the whole creek, however, revealed that there were two species of Corophium present, each species having a different distribution. The aim of this paper, therefore, is to discover reasons for the differing distribution of these two species—Corophium volutator (Pallas) and Corophium arenarium (Crawford)--in Benfleet Creek. Position of the sampling stations Sampling stations were set up in each type of substratum in the Creek. On the sand banks A, B and E—C. arenarium areas—random samples were taken. Their position is shown on figure 2 by small circles. In addition, a transect was put down on bank B and sampling stations were set up at 1 yard intervals from Low Water Mark on August 7th, 1959 (see figure la). The position of this transect is shown in figure 2 by a line across area B. Nine other transects 20 feet apart were also surveyed and the positions of the C. arenarium zone marked. Some of these transects are used in figure 6. Another transect was put across area G, the C. vo/utator zone, about mile below Benfleet Bridge. The transect was surveyed (figure lb) and four sampling stations established 8, 50, 115 and 125 yards from H.W.M.M.T. The position of the transect is shown by the line across area G in figure 2. Three sampling stations (shown by crosses in figure 2) which have been called the Long Transect were set up outside the estimated Corophium zones. It was important that this should be done in order to complete the ecological picture and to discover factors limiting the distribution of Corophium. Animal communities are subject to rapid changes, caused by the yearly life cycle of the species, Watkins (1941), and possible changes in the location of the community at different seasons of the year. Thus the time at which any investigation of an animal community is made is of some importance. Two preliminary investigations of the area were carried out, one at the end of June and the other at the end of July. The detailed work was done during the period of August 1st to August 29th, 1959. Final details, mainly concerned with collecting water and some soil samples were completed up to September 7th and some soil samples had to be recovered and the pH of the water determined early in December. Thus the details of distribution of the species, and the conclusions arrived at as to the factors determining their distribution, can only be related to the month of August 1959, 294 TIIE ESSEX NATURALIST and further work, completed over a number of years would be necessary if these conclusions were to be confirmed. METHODS Specimen Sampling—In two areas, B and G, transect sampling was used. All other samples taken were random samples. Each sample covered an area of 100 sq. ems. to a depth of 15 ems. The figures in Table IV are obtained from two such samples at each station. Water Content—A core sample of the substratum at each station was obtained by using a glass tube 5/8th of an inch in diameter and 10 inches long. This was screwed into the substratum and a rubber bung inserted in the top of the tube. The tube was then screwed out again and another bung inserted in the other end, Fraser (1932), Stopford (1951), Southward (1953). The water content of one inch of the core from half inch below the surface was estimated in the normal way. Soil Analysis—Core samples were obtained in the same way as above, air dried and analysed later in the laboratory. The method described by Rees (1940) was found to be unsatisfactory and a more refined method, in which subsamples are taken from a soil suspension by pipette, was therefore used. This method will be described in detail in another paper. Salinity—The amount of sodium chloride was estimated by the usual method of titrating — silver nitrate against 10 mls. 10 of seawater to which a few drops of potassium chromate had been added. Total salt content was obtained by consulting Hydro- graphical tables—Knudsen (1901—reprint 1953). PHYSICAL AND CHEMICAL DATA Salinity TABLE I Urns Urns Total salt Location Tide Cl I Ltr. NaCl I Ltr. contenb in, 0100 Leigh Island High 17.16 28.30 31.00 Low 15.92 26-35 28.77 Benfleet Bridge High 16.66 27-42 30.05 Low 15.22 24.45 27.50 Westcliff Front High 17.83 29.40 32-21 The above table shows that there is little variation in the salinity of the water with distance up the Creek, just over 1 ° / oo 0 at low water and just less than 1 /00 at high water.

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